Cryogenic purification of biogas with withdrawal at an intermediate stage and external solidification of carbon dioxide

ABSTRACT

A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, including a mixing means, a compressor, a first exchanger, a distillation column, a second exchanger, a separating means, an expanding means, and a separator vessel. Wherein, the mixing means is configured such that the recycle gas is the overhead vapour stream, and the first exchanger and the expanding means are combined.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French Patent Application No. 2106091, filed Jun. 9, 2021, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a plant and a process for producing liquid methane and liquid carbon dioxide from a biogas stream.

Biogas is the gas produced during the degradation of organic matter in the absence of oxygen (anaerobic fermentation), also known as methanization. This may be natural degradation—it is thus observed in marshland or in household waste landfills—but the production of biogas may also result from the methanization of waste in a dedicated reactor referred to as a methanizer or digester.

By virtue of its main constituents—methane and carbon dioxide—biogas is a powerful greenhouse gas; at the same time, it also constitutes a source of renewable energy that is appreciable in the context of the increasing scarcity of fossil fuels.

Biogas predominantly contains methane (CH₄) and carbon dioxide (CO₂), in proportions which can vary according to the way in which it is obtained, but also contains, in smaller proportions, water, nitrogen, hydrogen sulfide, oxygen, and also other organic compounds, in trace amounts.

Depending on the organic matter that has been degraded and on the techniques used, the proportions of the components differ, although on average biogas includes, on a dry gas basis, from 30% to 75% methane, from 15% to 60% CO₂, from 0 to 15% nitrogen, from 0 to 5% oxygen and trace compounds.

After a step of pretreating these contaminants, the biogas can be used as is to supply a boiler or a cogeneration unit, or else purified to obtain a gas which meets the specifications for injection into the natural gas network (e.g.: 3% CO₂ max).

In numerous regions of Europe and throughout the world, the natural gas network is not always accessible close to the areas of production of fermentable waste. Furthermore, while there is no need for heat at the biogas production site, depending on the purchase price of electricity, cogeneration does not always have a sufficient output to render profitable the major investment in a digestion unit. It is then advantageous in these two cases to transport the biogas to a distribution or consumption point. The liquefaction of biogas after purification would make it possible to transport biomethane at a lower cost. According to the regulations in certain geographic zones, it is forbidden to release CH₄ into the environment; this adds an additional constraint and restricts the choice of biogas separation processes to highly effective methods.

Today, biogas purification processes are mainly based on absorption, permeation or adsorption techniques. These systems then require the addition of a supplementary module in order to obtain biomethane in the liquid form. Moreover, in the majority of cases, the content of CO₂ in the biogas at the end of this purification step is still too high to supply such liquefaction systems.

A system of cryotrapping based on the principles of reversible exchangers has been proposed. This system is based on the solidification of the CO₂ present in the biogas on a cold surface (trapping), followed by a step of sublimation or liquefaction of the CO₂ using a hot source. For a continuous production of biomethane, is then necessary to work with several exchangers in parallel. Their solution makes it possible to separate and liquefy the methane and the CO₂ in two separate steps, but it is not possible to recover the cold used in the solidification of the CO₂.

Starting from there, one problem that arises is that of providing a method of separating and liquefying methane and CO₂ from biogas with a minimum loss of methane and using a minimum number of operations.

SUMMARY

One solution of the present invention is a combined plant for cryogenic separation and liquefaction of methane and carbon dioxide comprised in a biogas stream, comprising:

-   -   a means M1 for mixing the biogas 1 with a recycle gas R,     -   a compressor for compressing the mixture to the distillation         pressure,     -   an exchanger E01 for cooling the compressed mixture,     -   a distillation column K01 supplied with the cooled mixture and         making it possible to produce methane at the top of the column         and a CO₂-enriched liquid at the bottom of the column,     -   an exchanger E02 for liquefying the methane produced at the top         of the column,     -   a means M2 for separating the liquefied methane into two         portions: a “reflux” portion 6 and a “product” portion 5,     -   a means M3 for expanding and heating the CO₂-enriched liquid         recovered at the bottom of the column and for recovering the         cold from the CO₂-enriched liquid, and     -   a separator vessel V01 for receiving the CO₂-enriched stream         from the means M3 and for recovering an overhead vapour and         liquid CO₂ 7,     -   a means for withdrawing vapour V1 from an intermediate stage of         the distillation column K01,     -   a means for partially condensing the vapour V1 and producing a         two-phase stream D1,     -   a means for reinjecting the two-phase stream D1 into the         distillation column K01 at the stage corresponding to the         equilibrium temperature,

with

-   -   the means M1 such that the recycle gas R corresponds to the         overhead vapour recovered at the outlet of the separator vessel         V01,     -   the exchanger E01 and the means M3 being combined, and     -   the cryogenic separation unit comprising:     -   a distillation column K01 comprising a cold section 2 at the top         of the column and a hot section 3 at the bottom of the column,     -   a means 4 for physically separating the cold section 2 and the         hot section 3,     -   at least two containers V04 A/B external to the distillation         column, for placing in contact the liquid from the cold section         and the vapour rising from the hot section and for trapping all         the solid CO₂, and     -   a device for regenerating the external containers, comprising a         means for extracting a fluid from the distillation column K01         which is capable of liquefying the solid CO₂, a means for         introducing this fluid into the external containers in         regeneration so as to bring about the melting of the solid CO₂         and a means for reintroducing the resulting liquid CO₂-vapour         mixture into the distillation column K01.

Depending on the case, the plant according to the invention may have one or more of the following characteristics:

-   -   the means for partial condensation is the exchanger E01;     -   the plant comprises, upstream of the means M1, means for drying         and desulfurization of the biogas;     -   the plant comprises, upstream of the means M1, a means C01 for         compressing the biogas to the pressure of the recycle gas R;     -   the plant comprises, upstream of the means M1, a means C01E         and/or C02E for cooling the biogas to ambient temperature;     -   the exchanger E02 is comprised in a closed refrigeration         circuit;     -   the refrigeration circuit uses methane as refrigerant fluid;     -   the distillation column K01 comprises heating at the bottom of         the column.

The present invention also relates to a combined process of cryogenic separation and liquefaction of methane and carbon dioxide comprised in a biogas stream, using the plant as defined previously, and comprising:

-   -   a) a step of mixing the biogas 1 with a recycle gas R,     -   b) a step of compressing the mixture to the distillation         pressure,     -   c) a step of cooling the compressed mixture in the exchanger         E01,     -   d) a step of distilling the cooled mixture in the distillation         column K01 so as to produce methane at the top of the column and         a CO₂-enriched liquid at the bottom of the column,     -   e) a step of liquefying the methane produced at the top of the         column in the exchanger E02,     -   f) a separation step for separating the liquefied methane into         two portions: a “reflux” portion 6 and a “product” portion 5,     -   g) a step of expanding and heating the CO₂-enriched liquid         recovered at the bottom of the column in the exchanger E01, and         of recovering the cold from the CO₂-enriched liquid, and     -   h) a step of separating the CO₂-enriched stream resulting from         the exchanger E01 in the separator vessel V01 into liquid CO₂ 7         and overhead vapour,

with the recycle gas R corresponding to the overhead vapour produced in step a), and the distillation step d) comprising the following sub-steps:

i) a sub-step of introducing the methane-carbon dioxide mixture into the bottom of the column,

ii) a sub-step of removing the liquid from the cold section 2 of the distillation column,

iii) a sub-step of removing the vapour from the hot section 3 of the distillation column,

iv) a sub-step of introducing the liquid and the vapour removed from steps b) and c) into at least one container external to the distillation column K01 so as to place them in contact and to trap the solid CO₂,

v) a sub-step of regenerating at least one external container, comprising a sub-step of extracting a fluid from the distillation column K01 which is capable of liquefying the solid CO₂, a sub-step of introducing this fluid into the external container(s) in regeneration so as to bring about the melting of the solid CO₂ and a sub-step of reintroducing the resulting liquid CO₂-vapour mixture into the distillation column K01,

vi) a sub-step of recovering methane at the top of the column and a CO₂-enriched liquid at the bottom of the column,

vii) a sub-step of withdrawing vapour V1 from an intermediate stage of the distillation column K01,

viii) a sub-step of cooling the vapour V1 in the exchanger E01 so as to partially condense this vapour and to produce a two-phase stream D1, and

ix) a sub-step of reinjecting the two-phase stream D1 into the distillation column K01 at the stage corresponding to the equilibrium temperature.

Note that the withdrawal of vapour V1 then the reinjection of a two-phase stream D1 as described above makes it possible to reduce the reflux at the top of the column which is at a much lower temperature level and is supplied directly by an external source of cold.

Depending on the case, the process according to the invention may have one or more of the features below:

-   -   the process comprises, upstream of step a), drying and         desulfurization steps;     -   the process comprises, upstream of step a), a step of         compressing the biogas to the pressure of the recycle gas R;     -   the process comprises, upstream of step a), a step of cooling         the biogas to ambient temperature;     -   the process comprises, downstream of step h), a step of heating         the liquid CO₂ so as to vaporize it;     -   the heating of the liquid CO₂ is performed in the exchanger E01;     -   step e) is performed by cooling the produced methane by means of         a refrigerant fluid;     -   in step b), the mixture is compressed to a pressure of between 7         and 46 bar;     -   in sub-step v., the fluid extracted from the distillation column         is vapour obtained from a lower section at a higher temperature         of the distillation column K01;     -   sub-step iv. is performed with control of the temperature in the         external container;     -   in sub-step v., at least two external containers are regenerated         in series.

The process according to the invention makes it possible to separate and liquefy the products of the biogas in a single combined distillation/liquefaction operation. The operating conditions of the products at the inlet and outlet of the column and in the recycle section have been calculated to prevent the formation of solid CO₂.

The thermal integration between the streams of the separation section and those of the refrigeration cycle enable the recovery of the cold used in the liquefaction of the CO₂ and in the recycling of the liquid methane. It is possible to completely or partly recover the energy used in the liquefaction of the CO₂ if this CO₂ is not desired as a product or when it can be used in the gaseous state.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention.

FIG. 2 illustrates further details of FIG. 1 , in accordance with one embodiment of the present invention.

FIG. 3 illustrates further details of FIG. 1 , in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The pretreated biogas 1 (pretreated by drying, desulfurization) is introduced into the process at atmospheric pressure and temperature, it is compressed a first time in a compressor C01, to the pressure of the recycle circuit (around 8 bar). After compression, it is cooled in C01E to ambient temperature with CW (=Cooling Water) or air.

Next, it is mixed with a recycle stream R, the mixture is compressed in a compressor C02, to the pressure of the distillation column (around 15 bar) or more depending on the requirements of the downstream exchanger E01 and it is cooled to ambient temperature in C02E, with CW or air.

Preferably, C01E and C02E are shell and tube exchangers (coolers of the compressors).

The mixture of biogas—recycle stream R is sent to the exchanger E01. The main purpose of this exchanger is to cool the mixture in preparation for the distillation. The mixture can then be expanded or supplied directly to the column where it will be used as reboiler.

If there is no heat source at the bottom of the column, it is necessary to inject the mixture into the bottom to ensure the circulation of vapour from the bottom. If there is a heat source in the bottom of the column (reboiler), the mixture is introduced higher up in the column.

The distillation column KOI separates the methane from the carbon dioxide. The feed for the column is the biogas+recycle stream R mixture. This feed acts as main reboiler; an additional source of heat may also be used (for example an electrical resistance heater, vapour or a portion of the hot biogas in indirect contact).

The distillation column K01 comprises a cold section 2 at the top of the column and a hot section 3 at the bottom of the column and a means 4 for physically separating the cold section and the hot section.

The external containers V04 A/B of the distillation column K01 operate in alternating sequences of filling and regeneration, with always at least one container being filled to allow continuous operation of the column, without said column being disturbed by the presence of solids. The formation of solids in the containers will be brought about by controlling the temperature. The CH₄-rich cold liquid L coming from the top of the distillation column K01 will have a lower temperature than the equilibrium temperature of CO₂ solidification, it will be brought into contact with the rising vapour V (mixture with similar concentrations of CH₄ and CO₂), the temperature of which is higher than the equilibrium temperature of CO₂ solid formation (cf. FIG. 2 ). The contact of the two fluids will result in a liquid-vapour mixture whose concentrations and temperature are in the range for formation of solid CO₂F. The rate of saturation of the container with solids will depend on the process pressure and on the size of the containers.

Melting (regeneration) of the solid CO₂ trapped in the external containers will take place by direct contact with a stream of vapour extracted from a lower section of the column at a higher temperature. The resulting liquid-vapour mixture M will be reinjected into a lower stage of the column. And the overhead vapour T will be reinjected into the top of the column.

At any time, there will be at least two regeneration containers in series (cf. FIG. 3 ). The first container R1 will be the one providing the main part of the cold, the second container R2 will ensure that the temperature and the liquid fraction of the mixture M′ to be reinjected in the column will remain constant in spite of the depletion of solid E in the first container. Once the first container has been completely regenerated, it can be put in service for the formation of solids. The second container will then act as a cold supply and the newly saturated container will be put into regeneration.

Vapour V1 is withdrawn from an intermediate stage of the distillation column K01. This vapour V1 is sent into the exchanger E01 and partially condensed so as to produce a two-phase stream D1. The two-phase stream D1 is reinjected into the column at the stage corresponding to the equilibrium temperature.

The product at the top of the column is pure CH₄ in the vapour state. The bottom product is a liquid rich in CO₂, containing around 95%-98%.

The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. A portion of the methane leaves the cycle as product and the other portion (reflux portion) is used as recycle for the column and reinjected into the top of the column.

The CO₂-enriched liquid recovered at the bottom of the column is expanded and heated in the exchanger E01 countercurrent to the biogas—recycle stream R mixture.

The CO₂-enriched stream from the exchanger E01 is sent to the separator vessel V01.

The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the stream previously named “recycle stream R”.

The liquid from the bottom of the vessel V01 is the pure CO₂. This can, depending on the requirements, leave the process as product or be reheated in the exchanger E01 and in another exchanger E03 of the refrigeration circuit in order to be completely vaporized before leaving the cycle. Note that the pure CO₂ could alternatively be reheated and vaporised in the exchanger E03 without passing through the exchanger E01.

The exchanger E01 therefore uses, as sources of cold: the CO₂-enriched liquid recovered at the bottom of the column, the overhead vapour from the vessel V01 named “recycle stream R” at the outlet of the exchanger E01, and optionally the pure liquid CO₂ recovered at the bottom of the vessel V01 in the case where the vaporisation thereof is desired.

The process requires an input of refrigeration power in order to operate. This input of cold is represented in FIG. 1 by the refrigeration circuit: it is composed of:

-   -   a compressor C03 with cooler C03E;     -   an exchanger E03 which cools the compressed fluid using the         recycled refrigerant fluid and the cold recovered from the         separation cycle;     -   a turbine ET01 and a JT valve PV05, for the expansion of the         refrigerant fluid and production of cold;     -   a separator vessel V02 separating the vapour and liquid phases         of the refrigerant fluid;     -   an exchanger E02 which uses the liquid phase of the refrigerant         fluid to liquefy the biomethane at the top of the distillation         column.     -   The refrigerant fluid used in the scheme is CH₄ but it can be         replaced by other fluids such as N₂, N₂+H₂, inter alia.

This refrigeration cycle can be replaced by other sources of cold (depending on the amount of liquid biomethane to be produced). By way of example, but not exclusively:

-   -   using a source of liquid nitrogen;     -   by a Brayton cycle process.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

What is claimed is:
 1. A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, comprising: a mixing means for mixing a biogas stream with a recycle gas stream, thereby producing a mixed biogas stream, a compressor for compressing the mixed biogas stream to a pressure suitable for distillation, thereby producing a compressed stream, a first exchanger for cooling the compressed mixture, thereby producing a cooled stream, a distillation column, comprising a top and a bottom, configured to be supplied with the cooled stream and configured to produce a methane stream at the top and a CO₂-enriched liquid stream at the bottom, aa second exchanger for liquefying the methane stream, thereby producing a liquefied methane stream, a separating means for separating the liquefied methane stream into two portions: a “reflux” portion and a “product” portion, an expanding means for expanding and heating the CO₂-enriched liquid stream, thereby producing a heated CO₂-enriched stream, and for recovering the cold from the CO₂-enriched liquid stream, and a separator vessel for receiving the heated CO₂-enriched stream and for recovering an overhead vapour stream and a liquid CO₂ stream, a means for withdrawing a vapour stream from an intermediate stage of the distillation column, a condensing means for partially condensing the vapour stream and producing a two-phase stream, a means for reinjecting the two-phase stream into the distillation column at a stage corresponding to an equilibrium temperature, wherein the mixing means is configured such that the recycle gas stream corresponds to the overhead vapour stream recovered at the outlet of the separator vessel, the first exchanger and the expanding means are combined, and the cryogenic separation unit comprises: a distillation column comprising a cold section at the top of the column and a hot section at the bottom of the column, a separation means for physically separating the cold section and the hot section, at least two containers external to the distillation column, configured to place in contact a liquid stream from the cold section and a vapour stream rising from the hot section and for trapping solid CO₂, and a device for regenerating the at least two external containers, comprising an extracting means for extracting a fluid from the distillation column which is configured to liquefy the solid CO₂, a means for introducing this fluid into the at least two external containers in regeneration thereby melting the solid CO₂ and a means for reintroducing the resulting liquid CO₂-vapour mixture into the distillation column.
 2. The plant according to claim 1, wherein the means for partial condensation is the first exchanger.
 3. The plant according to claim 1, further comprising, upstream of the mixing means, a drying means for drying and desulfurization of the biogas stream.
 4. The plant according to claim 1, further comprising, upstream of the mixing means, a compressing means for compressing the biogas stream to the pressure of the recycle gas stream.
 5. The plant according to claim 1, further comprising, upstream of the mixing means, a cooling means for cooling the biogas stream to ambient temperature.
 6. The plant according to claim 1, wherein the second exchanger is comprised in a closed refrigeration circuit.
 7. The plant according to claim 6, wherein the refrigeration circuit uses methane as refrigerant fluid.
 8. The plant according to claim 1, wherein the distillation column comprises heating at the bottom of the column.
 9. A combined process of cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, using the plant as defined in claim 1, and the process comprising: a) mixing the biogas stream with the recycle gas stream, b) compressing the mixed biogas stream to a pressure suitable for distillation c) cooling the compressed biogas stream in the first exchanger, d) distilling the cooled stream in the distillation column thereby producing a methane stream at the top of the column and a CO₂-enriched liquid stream at the bottom of the column, e) liquefying the methane stream, f) separating the liquefied methane into two portions: a “reflux” portion and a “product” portion, g) expanding and heating the CO₂-enriched liquid stream, and of recovering the cold from the CO₂-enriched liquid stream, and h) separating the CO₂-enriched stream resulting from the exchanger n the separator vessel into a liquid CO₂ stream and overhead vapour stream, with the recycle gas stream corresponding to the overhead vapour stream produced in step a), and the distillation step d) comprising the following sub-steps: i. a sub-step of introducing the methane-carbon dioxide stream into the bottom of the column, ii. a sub-step of removing a liquid stream from the cold section of the distillation column, iii. a sub-step of removing a vapour stream from the hot section of the distillation column, iv. a sub-step of introducing the liquid and the vapour removed from steps b) and c) into at least one container external to the distillation column thereby placing them in contact and to trap solid CO₂, v. a sub-step of regenerating at least one of the at least two external containers, comprising a sub-step of extracting a fluid from the distillation column which is configured to liquefy the solid CO₂, a sub-step of introducing the fluid into the at least two external containers in regeneration thereby melting the solid CO₂ and a sub-step of reintroducing the resulting liquid CO₂-vapour mixture into the distillation column, vi. a sub-step of recovering a methane stream at the top of the column and a CO₂-enriched liquid stream at the bottom of the column, vii. a sub-step of withdrawing a vapour stream from an intermediate stage of the distillation column, viii. a sub-step of cooling the vapour stream in the first exchanger thereby partially condensing the vapour and producing a two-phase stream, and ix. a sub-step of reinjecting the two-phase stream into the distillation column at the stage corresponding to an equilibrium temperature.
 10. The process according to claim 9, further comprising, upstream of step a), steps of drying and of desulfurization the biogas mixture.
 11. the process according to claim 9, further comprising, upstream of step a), a step of compressing the biogas stream to the pressure of the recycle gas stream.
 12. The process according to claim 9, further comprising, upstream of step a), a step of cooling the biogas stream to ambient temperature.
 13. The process according to claim 9, further comprising, downstream of step h), a step of heating and vaporizing the liquid CO₂ stream.
 14. The process according to claim 13, wherein the heating of the liquid CO₂ stream is performed in the first exchanger.
 15. The process according to claim 9, wherein step e) is performed by cooling the methane stream by means of a refrigerant fluid.
 16. The process according to claim 9, wherein in step b), the mixed biogas stream is compressed to a pressure of between 7 and 46 bar.
 17. The process according to claim 9, wherein in sub-step v., the fluid stream extracted from the distillation column is the vapour stream obtained from a lower section at a higher temperature of the distillation column.
 18. The process according to claim 9, wherein sub-step iv. is performed with control of the temperature of the external container.
 19. The process according to claim 9, wherein in sub-step v., at least two external containers are regenerated in series. 